Rocket motors employing solid propellants typically include a rigid outer casing or shell; a heat insulating layer (insulation) bonded to the inner surface of the casing; a liner layer (liner) bonded to the insulating layer; and a solid propellant grain bonded to the liner, the insulation is generally fabricated from a composition capable of withstanding the high temperature gases produced when the propellant grain burns, thus, protecting the case. The liner is an elastomeric composition which must bond the propellant grain to the insulation and to any uninsulated portions of the case, as well as inhibit interfacial burning.
Polyurethane liners, used in a large number of rocket motors, are very sensitive to variations in stoichiometry, i.e., the isocyanate/hydroxy equivalents ratio (NCO/OH). They are typically formulated at an NCO/OH ratio between 1.0 and 1.5. A slight excess of isocyanate (NCO) is required to compensate for curative diffusion into the insulation, the presence of moisture which is reactive with isocyanates, and other process variables which may reduce the isocyanate presence, the liner becomes very soft and/or does not cure at NCO/OH ratios of less than 1.0 or greater than 1.5. Process variables such as moisture contamination, relative humidity, ingredient migration (diffusion), weighing errors, misformulation and other parameters which may directly or indirectly affect the NCO/OH ratio can greatly alter liner properties. NCO/OH ratio variations as small as 0.10 from nominal can result in a soft, degraded propellant to liner to insulation bondline, bond failure and potential motor malfunction. Very rigorous process controls must therefore be imposed with polyurethane liners in order to maintain critical bondline integrity.
A second function of the liner is to inhibit the burning surface of the propellant grain when the interface is exposed to the flame front. There are several basic propellant grain configurations. The two most commonly used configurations are the center perforated grain and the end burning grain. In the center perforated grain configuration, the flame front advances radially from the center perforation to the outer casing. The insulating layer and liner are not exposed to the flame front or hot gases until near the end of motor firing in this configuration. In the end burning grain configuration, the flame front advances axially from the nozzle end of the motor to the forward dome. The insulation and liner are directly exposed to the hot combustion gases as the flame front advances in this configuration. The insulation in the aft section of the motor has the longest exposure time and the insulation in the forward section has the least exposure time.
End burning propellant grains are particularly sensitive to interfacial burn rate gradients since the flame front advancement is perpendicular to the interface. These interfacial burn rate gradients cause the propellant to burn at a different rate near the liner bondline. Interfacial ballistics are complex in nature but are believed to be a function of several factors including propellant ingredient diffusion, particle alignment and/or particle size stratification during propellant casting, and localized radiant/convective heat transfer conditions at the liner interface.
Propellant ingredient diffusion into the liner changes the propellant composition and therefore the burn rate at the liner interface. Curative and plasticizer diffusion, for example, results in a higher solids and therefore a higher burn rate propellant at the interface. Conversely, burn rate catalyst diffusion results in a catalyst deficient and, therefore, a slower burning propellant at the interface. Particle size distribution changes at the interface and preferential alignment of solid particulate ingredients with a length to diameter ratio greater than one during propellant casting typically result in a propellant burn rate gradient near the liner interface. Localized heat transfer conditions can also affect propellant temperature and burn rate at the interface.
It is undesirable to have large and uncontrolled propellant burn rate gradients in a rocket motor design. Burn rate gradients result in progressive changes in propellant burning surface area, chamber pressure, motor thrust and, in the case of end burning grains, unpredictable insulation exposure times. Motor design and performance are therefore somewhat unpredictable and exhibit a high degree of variability. In extreme cases, severe burn rate gradients have resulted in motor malfunction.
From the foregoing, it would be a significant advancement in the art to provide a solid rocket motor propellant liner which is insensitive to large variations in stoichiometry, which is insensitive to adverse process conditions including high relative humidity, residual moisture in liner ingredients and residual moisture in the insulation substrate, and which is able to modify and/or control the ballistic properties of the adjacent interfacial propellant layer. It would also be a significant advancement in the art to provide an in situ labeling technique for liner application monitoring and control.
Such solid rocket motor propellant liner compositions are disclosed and claimed herein.